How double cousins happen
Say two brothers marry two sisters. Each couple has kids. Those kids aren't just cousins through their dads, they're cousins through their moms too. Same grandparents on both sides. That makes them double first cousins.
It also happens when a brother and sister from one family marry a sister and brother from another. Same result: every child in both families shares all four grandparents.
In close-knit towns, immigrant communities and big families, this used to be common. Two families who got on well often married into each other more than once.
How much DNA do double cousins share?
| Relationship | Shared DNA (average) | Shared grandparents |
|---|---|---|
| Full siblings | 50% | All four (and both parents) |
| Half siblings | 25% | Two (and one parent) |
| Double first cousins | 25% | All four |
| First cousins | 12.5% | Two |
| Second cousins | 3% | None (they share great-grandparents) |
Because double first cousins share about as much DNA as half siblings, grandparents or aunts and uncles, DNA tests often suggest one of those instead. One clue tells them apart: double cousins can share small fully identical regions, stretches matching on both copies of a chromosome, which half siblings almost never do. Ages and family history do the rest. Check your shared cM.
Double second cousins and other combinations
The same thing can happen one generation further back. If your parents are double first cousins to their parents, you can end up as double second cousins, sharing about twice the usual 3%. Any time two families intermarry more than once, the DNA adds up, which is why DNA tests sometimes overestimate how closely people are related in tight communities.
Can double cousins marry?
Where first cousins can't marry, double first cousins can't either. Where first cousins can, the law usually treats double cousins the same way. The exception is North Carolina, which allows first cousins to marry but specifically bans double first cousins. Check every state.
What about kids?
Double first cousins share twice the ancestry of ordinary first cousins, so they're more likely to carry the same hidden recessive variant from a shared grandparent. In genetic terms they're about as related as half siblings. If double cousins are planning a family, a genetic counselor and a carrier screen for both partners are strongly recommended.
This article is general information, not legal or medical advice.